It has been known that diamond semiconductor has quite a large band gap of about 5.5 eV (about 225 nm in terms of light wavelength) at room temperature and, thus, can be an insulator when it is in an intrinsic state not containing dopant (impurity). In order to grow its single crystal thin film, a microwave-excited plasma chemical vapor deposition method using an atmosphere substantially containing carbon and hydrogen, such as CH4 (methane) and H2 (hydrogen) gases, has been developed (Patent Document 1) and widely applied. In the microwave-excited plasma chemical vapor deposition, p-type (in which holes are main carrier) conductivity is often controlled by adding B (boron) as dopant.
It is known that since the microwave-excited plasma chemical vapor deposition uses an atmosphere containing hydrogen, the surface of the resulting diamond single crystal film is substantially covered with hydrogen. More specifically, it is known that a C—H molecular structure formed by terminating unbonded hands of carbon atoms (C) with hydrogen atoms (H) (hereinafter referred to as “hydrogenation”) is present at the surface, and that a conductive surface layer in which holes acting as the main carrier are localized close to the surface (at a depth within 2 nm) is formed inside the diamond close to the surface of the diamond by the hydrogenation. It is also known that undoped or boron-doped (100) and (111) plane single crystal thin films and polycrystalline thin films have such an electroconductive surface layer.
The mechanism how the conductive surface layer is formed is being actively discussed in the world, and it has been found at least according to experimental studies that the conductive surface layer is: (1) stable up to about 200° C.; and (2) formed only in the hydrogenated diamond surface. It has been also found that the conductive surface layer disappear by treatment (oxidation) with a solution removing the bonded hydrogen from the surface, for example, by immersion in a boiling mixed solution of sulfuric acid and nitric acid. The inventors of the present invention have confirmed this.
For optical sensor elements that detect UV light irradiating a light-receiving portion according to the changes in electrical resistance or photo-induced current of the light-receiving portion, it has been proposed that Si semiconductors capable of detecting visible light having a wavelength in the range of 400 to 650 mm, or AlxGa1-xN semiconductors (0≦X≦1) or diamond semiconductors having no detection sensitivity to the visible light or optical noises in the infrared region are used as solid materials of the light-receiving portion.
In the optical detection principle of such optical sensor elements, electron-hole pairs are formed in the semiconductor of the light-receiving portion by irradiating the semiconductor to light having an energy more than or equal to the band gap, and the electron-hole pairs being carriers change the electrical resistance and photo-induced current of the semiconductor. The sensor detects these changes. Accordingly, the sensor element can have a simple two-terminal structure in which two electrodes are bonded to the semiconductor, and, thus, the resulting UV sensor can be extremely simple.
Widely used optical sensor elements having a two-terminal element include a metal-semiconductor-metal (MSM) element having a comb-like electrode structure and a Schottky element having two types of electrodes: a rectifying electrode through which light is detected and an ohmic electrode.
Non-Patent Document 1 describes an example of the UV sensor element using a diamond semiconductor. This UV sensor element is a MSM type photoconductive sensor element including a light-receiving portion defined by a conductive surface layer of a polycrystalline diamond thin film, and a Ti first layer electrode and a Au second layer electrode, and has a detection sensitivity of 0.03 A/W to UV light of 200 nm.
Non-Patent Document 2 describes an MSM type photoconductive sensor element including a light-receiving portion made of a polycrystalline diamond film from which the conductive surface layer has been removed by oxidation, and a Ti first layer electrode and a Au second layer electrode. This MSM type photoconductive sensor element has a detection sensitivity of 0.02 A/W to UV-light of 200 nm.
Non-Patent Document 3 describes a Schottky sensor element including a Au rectifying electrode and a Ti/Ag/Au ohmic electrode (slash “/” represents deposition order) that are formed on a polycrystalline diamond thin film. Although the absolute value of the detection sensitivity of this element is unknown, it has been reported that the ratio of the detection sensitivity to light having a wavelength of 200 nm to the detection sensitivity to light having a wavelength of 600 nm (referred to as visible light blind ratio) is five digits.
Patent Document 2 has disclosed prior art relating to a diamond UV sensor including a light-receiving portion made of a 40 μm thick polycrystalline or (100) or (111) oriented diamond thin film having a surface from which bonded hydrogen has been removed. The detection sensitivity of this sensor is not insufficient for practical use. Patent Document 3 has disclosed a diamond UV sensor element including a light-receiving portion defined by a conductive surface layer of diamond. This element can detect light having wavelengths in the entire visible region, and is a photoconductive sensor element using the defect level of the diamond band gap. Hence, it cannot selectively detect UV light of 250 nm or less.    Patent Document 1: Japanese Examined Patent Application Publication No. 59-27754    Patent Document 2: Japanese Unexamined Patent Application Publication No. 11-248531    Patent Document 3: Japanese Unexamined Patent Application Publication No. 11-097721    Non-Patent Document 1: H. J. Looi, M. D. Whitfield, and R. B. Jackman, Appl. Phys. Letts. 74, 3332 (1999)    Non-Patent Document 2: R. D. McKeag and R. B. Jackman, Diamond Relat. Mater. 7, 513 (1998)    Non-Patent Document 3: M. D. Whitfield, S. S M. Chan, and R. B. Jackman, Appl. Phys. Lett., 68, 290(1996)